Thread: Geometry of woodturning

Nothing like a bit of empirical research Ern! Good point.

It is interesting in this context to look at how planes such as those made by Terry Gordon perform on tough timbers when used with the blade set at a high cutting angle. The results with a sharp blade are brilliant. Just a question though
:When does cutting become scraping?

How is sheer scraping differently defined to sheer cutting?

Whar influence do cutting tools' metalurgy have one the angles at which they are sharpened and at which they cut most effectively?

So many questions, so few answers.

Yes, good questions.

Terry's high angle planes (also Muji, and LN and V. bevel up planes with high cutting angles as well) are designed to lift and break chips on interlocked grain before they can be lifted below the intended cutting line. You can attempt a similar effect with a Stanley if you put say a 10* back bevel on the blade but these blades are prone to flex and don't yield a good finish.

FWIW I regard scraping as a special case of cutting.

Others more expert than me will of course provide better opinion.

As for metallurgy, that's a book in itself. At a guess finer particles such as you get with powder metallurgy or cryo treatments provide a finer cutting edge (ie. less jagged with the tips less likely to break off) so more acute bevel angles can be used. That must mean less fibre crushing and earlier fibre severing in the sever/lift process.

Originally Posted by rsser

We almost inevitably will be cutting long grain but against the way it lies from time to time as well with resulting tear-out.
...
...while the geometry is clearly important there's other factors in the mix.

I am calling geometry also finding the angle between the blade and the direction of the grain - which changes at any given point of a curve and at opposite points of the rotation if turning with the grain. Hope this does not create problems of semantics...

Artme's question is an example of the things I would like to settle vith a definition: for me, cutting becomes scraping when the angle between the upper side of the blade and the surface of the wood becomes smaller than 90 degrees. There can be 1000s of descriptions, but this definition, if correct, should be true for all of them. And would deny that scraping "is a special case of cutting" because it means that its purpose is to break and burnish the fibres, not to cut them.

Originally Posted by rsser

FWIW I regard scraping as a special case of cutting.

Agreed, it's a scraping cut.

Originally Posted by Frank&Earnest

.... cutting becomes scraping when the angle between the upper side of the blade and the surface of the wood becomes smaller than 90 degrees.

Agreed, in a scraping cut the axis of the cutting face of the blade is presented at 90<meta http-equiv="Content-Type" content="text/html; charset=utf-8"><meta name="ProgId" content="Word.Document"><meta name="Generator" content="Microsoft Word 9"><meta name="Originator" content="Microsoft Word 9"><link rel="File-List" href="file:///C:/DOCUME%7E1/BRON%26N%7E1/LOCALS%7E1/Temp/msoclip1/01/clip_filelist.xml"><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <woNotOptimizeForBrowser/> </w:WordDocument> </xml><![endif]--><style> <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:612.0pt 792.0pt; margin:72.0pt 90.0pt 72.0pt 90.0pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> </style>° or less to the tangent that is formed at the point of blade/wood contact, where the tangent is aligned to the direction of blade travel. Skewing the face of the blade skews the tangent away from the direction of blade travel and, in effect, reduces the cutting angle without changing the axis angle.

Perhaps an accurate definition but not one that I would throw at a novice woodturner to confuse them or that useful for an experienced turner who knows all this intuitively from experience without thinking about it in this way, unless they were into designing their own woodturning tools....

.....

No argument with your assessment of usefulness, Neil, I believe I said that much myself, using different words...

I am indeed confused by what you deem to be an expansion of my definition, though: whatever the woodturning skill of the reader the complexity of language would be daunting for many like me, who are still unsure about the correct terminology. Thanks for telling me that the "upper side" is actually the "cutting face", for example.

For "axis of the blade" I assume you are referring to the axis of symmetry, that is in common language the centre line along the shaft of the tool. If that is the case, there is a piece missing, both in your and my previous imprecise definition: the direction of such line in relation to the vertical and horizontal axis. I understand your definition of the angle that has to be <90o as the angle you hold the tool in relation to the (horizontal) axis of rotation of the lathe. Which to me is still cutting (which explains why you also consider it cutting) unless the definition of scraping is met: that the angle formed by the axis of the blade (whatever its orientation relative to the horizontal axis) and the vertical axis delineated by the circumference or the base of the rotating cylinder at the tangent point is <90 degrees.

In summary, thank you for teaching me the correct terminology and helping me refining the definition... but are we still agreed now?

Originally Posted by Frank&Earnest

I am indeed confused by what you deem to be an expansion of my definition, though: whatever the woodturning skill of the reader the complexity of language would be daunting for many like me, who are still unsure about the correct terminology. Thanks for telling me that the "upper side" is actually the "cutting face", for example.

I have no wish to get into a terminology war, just using the most commonly used terms that I'm familiar with to describe what I understand to be happening.

For "axis of the blade" I assume you are referring to the axis of symmetry, that is in common language the centre line along the shaft of the tool.

Yes

If that is the case, there is a piece missing, both in your and my previous imprecise definition: the direction of such line in relation to the vertical and horizontal axis. I understand your definition of the angle that has to be <90o as the angle you hold the tool in relation to the (horizontal) axis of rotation of the lathe.

No, and that has highlighted a flaw in my definition. Oh, why in the heck did I let myself get into this....

The phrase 'blade travel' can be interpreted in two ways. A bad thing in any definition! I was referring to the direction the wood travels past the blade, as it does in woodturning. A more precise way to say this is 'where the tangent is aligned to the direction of wood rotation'.

So, not <90 to the axis, but <90 to the tangent aligned to the rotation of the wood.

Sorry about causing confusion there.

... but are we still agreed now?

Yes, I think so, if we take the following words from your second last paragraph.

.... the angle formed by the axis of the blade... and the circumference .... of rotating... at the tangent point is <90 degrees.

Think I might take a Bex and have a good lie down the next time I get the urge to get into definitions .....

.....

Originally Posted by NeilS

Yes, I think so, if we take the following words from your second last paragraph.

.... the angle formed by the axis of the blade... and the circumference .... of rotating... at the tangent point is <90 degrees.

Think I might take a Bex and have a good lie down the next time I get the urge to get into definitions .....

.....

Sorry to have caused you such discomfort, Neil, your contribution to this discussion is highly valuable. Next time we meet I'll offer you some more enjoyable liquid relaxant than Bex.

So Ern, given this definition, do you still think that scraping is a form of cutting?

Frank, I don't care much whether it is or isn't. There's a place for it, like cutting, which is to remove stock and to do so with maximum efficiency and minimal adverse effects.

Seems to me we've got a bit off topic.

Re geometry, there's the what and the why.

I think I have half a grip on the what and much less on the why.

Think I still have your email address Frank and will send you a scan of the what, acc. to one leading maker of bevel shaping machines ;-}

Thanks Ern. With that and the books there should be more than enough to chew.

My heartfelt thanks, Bookend, Rsser and NeilS, for your advice.

I know it is not the bloke's way, but in these two months I did read the instructions.
First of all, this exercise has proven the real value of the forum: to point out the right sources, not to waffle about opinions. Had I known before about Mike Darlow's book, I would not have had to reinvent the definition of scraping, (the good thing is at least that I did not make a fool of myself, for once,) I would just have answered Artme's question with: "as Mike Darlow says on page 42..."

This is indeed the most technical of the books addressed to the hobby woodturning public that I have seen, it answers most of the questions. The single point that I have found most helpful for my understanding is not there, though, but at least the practical descriptions of the book confirm its validity, so I feel confident expressing it.

What helped me making sense of how blade and wood interact is remembering a fact learned in primary school: that two lines intersect in one point. Whatever the shape of the tool and the wood, the circumference of the rotating piece and the edge of the tool are lines. Knowing where the point of contact is goes a long way towards understanding how the tool behaves: for example why the points of the skew or the wing of a gouge are harder to control than the axial centre of the tool.

Hopefully I will be able to use my recent learning to explain my theories about tool design...

Glad to hear you have sorted that out, Frank.

I'll get you to sort me out as well on all of that, if you can, the next time we get together...

.....

Glad to do that, Neil. Will be a good opportunity to swap observations about our own Dunning–Kruger effects...

As for Mssers Dunning and Kruger I have no difficulties at all!!

I understand cognition and metacognition very well Just can't figure some thing out!

Originally Posted by frank&earnest

glad to do that, neil. Will be a good opportunity to swap observations about our own dunning–kruger effects...:u

Originally Posted by artme

as for mssers dunning and kruger i have no difficulties at all!!

I understand cognition and metacognition very well just can't figure some thing out!:d

:d

.....